1. Field of the Invention
The present invention relates to an exposure apparatus, exposure method, and process for production of a device, more particularly relates to an exposure apparatus, exposure method, and process for production of a device in so-called step-and-scan exposure able to prevent defocus at the time of exposure at a periphery of a substrate and, further, superior in productivity.
2. Description of the Related Art
In the lithographic process which is one of the steps in the production of a semiconductor device, as the exposure apparatus for transferring a pattern formed on a mask (including a reticle) onto a wafer coated with a photoresist, frequent use is being made of a step-and-repeat exposure apparatus for reducing and projecting the image of the mask pattern onto a shot area of the wafer.
Recently, further, use has been made of a step-and-scan exposure apparatus for making a mask and wafer move synchronously with respect to a projection optical system while reducing and projecting part of a pattern on the mask through a projection optical system onto the wafer at a slit-like illumination area for exposure. This type of exposure apparatus has the advantage over the so-called step-and-repeat exposure apparatus that there is no increase of the load on the projection optical system and the pattern transferred can be made larger in area.
This step-and-scan projection exposure apparatus illuminates a slit-like area of the wafer and makes the substrate move relatively in the short direction of the slit-like area irradiated by the exposure beam for exposure. Further, immediately before exposure, it uses a sensor to find information on the position of the surface of the wafer in the direction of the optical axis (focus information) at the exposure position in a shot area to be exposed, continuously changes the posture of a stage carrying the wafer in accordance with the scanning motion, and controls the posture to keep the optimal image plane of the exposure apparatus in register with the surface of the wafer as much as possible at all times.
In such scanning exposure, when exposing a shot area at the center portion, that is, other than at the wafer periphery, the focus information of the wafer immediately before the start of exposure can be easily detected, so there is no problem.
When exposing a shot area at the periphery of the wafer, however, if scanning a slit-like area irradiated by the exposure beam starting from outside the wafer, the scan is started from a location where the position in the direction of the optical axis cannot be predicted, therefore there is the problem that exposure tends to be performed in a defocused state. Further, when exposing a shot area at the wafer periphery, the leveling control is insufficient, so there is the problem that defocus easily occurs at the time of exposure.
Therefore, when exposing a shot area at the wafer periphery, the method of relative scanning from the inside of the wafer to the periphery for exposure is sometimes used. With this method, however, it is necessary to move for scanning in a direction opposite from the previous shot area in only shot areas at the wafer periphery and there is therefore the problem that there is inevitably a decline in the throughput.
To solve these problems, as a means for judgement of whether to perform only focus control or leveling control based on the correspondence of the position of the wafer periphery and position of the shot area, the systems of Japanese Patent Application No. 9-247916 and Japanese Patent Application No. 10-32087 have been previously proposed by the present assignee. In these systems, only focus control or leveling control is performed when predetermined measurement positions of a focus detection sensor are reached to the wafer, but this is predicated on no switching of control once having started.
When scanning an exposure area from the inside of the wafer to the outside, leveling is possible and defocus can be prevented, but when scanning from the outside of the wafer to the inside, in many cases, leveling is not possible and only the focus ends up being controlled, so defocus easily occurs in a shot.